Multi-piece solid golf ball

ABSTRACT

The invention provides a multi-piece solid golf ball having a solid core, an inner cover layer and an outer cover layer, which outer cover layer has a plurality of dimples on a surface thereof. The value obtained by subtracting a material hardness (Shore D) of the outer cover layer from a material hardness (Shore D) of the inner cover layer is greater than −5 and less than +5. The dimples number at least 250 and not more than 500, have a surface coverage (SR) of at least 70% and a volume ratio (VR) of at least 1.0%, are of at least three types of mutually differing dimple diameter (DM) and/or dimple depth (DP), and have an average depth of at least about 0.18 mm and a diameter-to-depth ratio (DM/DP) of not more than about 23. The ball has a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is maintained at 60% or more of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000. This multi-piece solid golf ball is able to substantially reduce the distance traveled by the ball when struck at a high HS, while at the same time holding down the reduction in distance when struck at a low HS.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball having a solid core, an inner cover layer and an outer cover layer, and having a plurality of dimples on a surface of the outer cover layer. More specifically, the invention relates to a multi-piece solid golf ball which substantially reduces the distance traveled by the ball when struck at a high head speed (head speed is sometimes abbreviated below as “HS”) while at the same time undergoing little reduction in distance when struck at a low HS.

With recent advances in golfing equipment such as balls and clubs, golf balls have come to travel increasing distances. For this reason, to keep play fair, strict rules have been adopted which establish, in the case of a golf club, for example, the size of the head and the length of the shaft. Similarly, limitations have been placed on certain characteristics of the golf ball, such as its size, weight and initial velocity, so as to restrict excessive ball travel of the sort that would result in a loss of fair play.

The distance traveled by a golf ball is generally held down by limiting the initial velocity. However, in such cases, both at high head speeds and low head speeds, the distance traveled is often reduced in about the same ratio. As a result, such balls have significant drawbacks for low HS players.

As another approach, a variety of golf balls have been disclosed which, by optimizing the dimples on the surface of the ball, lower the flight trajectory and hold down decreases in distance.

For example, JP-A 05-103846 describes a golf ball in which the dimple diameter, dimple depth and number of dimples have been optimized. JP-A 10-043342 and JP-A 10-043343 disclose golf balls in which the amount of deformation by a ball when compressed under a load of 100 kgf has been optimized, along with which the dimple diameter divided by the dimple depth has been set to from 10 to 15 or the dimple space volume as a proportion of the total volume of a hypothetical sphere were the surface of the ball to have no dimples thereon has been set to from 0.7 to 1.1%. JP-A 2000-107338 discloses a practice golf ball in which the ball weight and diameter have been optimized.

In addition, JP-A 11-47311 and JP-A 11-47312 disclose solid golf balls in which an inner cover layer and an outer cover layer have the same or substantially similar Shore D hardnesses.

It is therefore an object of the present invention to provide a golf ball which can achieve a superior distance in a low HS range while holding down the distance traveled in a high HS range.

SUMMARY OF THE INVENTION

The inventors have conducted extensive investigations in order to achieve the above object. As a result, they have found that, in a multi-piece solid golf ball composed of a solid core, an inner cover layer and an outer cover layer, which outer cover layer has a plurality of dimples on a surface thereof, by specifying the size relationship between the material hardnesses (Shore D) of the inner cover layer and the outer cover layer; by specifying, for the dimples formed on the surface of the outer cover layer, the number of dimples, dimple surface coverage (SR), dimple volume ratio (VR), dimple types, average dimple depth, and dimple diameter-to-depth ratio (DM/DP); and by having the coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm maintained at 60% or more of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000, synergistic effects arising from dimple optimization and the suitable hardness relationship between the inner cover layer and the outer cover layer make it possible to substantially reduce the distance traveled by the ball when struck at a high HS while at the same time holding down the decrease in distance when the ball is struck at a low HS.

That is, unlike conventional methods of lowering the ball initial velocity or core initial velocity, the golf ball of the present invention is able, by combining low-trajectory dimples with the interior structure (multilayer structure) of the ball, to substantially reduce the distance of the ball when struck at a high HS while at the same time holding down to the extent possible, relative to the reduction in distance at high HS, the reduction in distance on low HS shots. As used herein, “distance” refers to the total distance traveled by a golf ball, including both the carry and the run.

Accordingly, the invention provides the following multi-piece solid golf balls.

-   [1] A multi-piece solid golf ball comprising a solid core, an inner     cover layer and an outer cover layer, which outer cover layer has a     plurality of dimples on a surface thereof, wherein a value obtained     by subtracting a material hardness (Shore D) of the outer cover     layer from a material hardness (Shore D) of the inner cover layer is     greater than −5 and less than +5; the dimples number at least 250     and not more than 500, have a surface coverage (SR) of at least 70%     and a volume ratio (VR) of at least 1.0%, are of at least three     types of mutually differing dimple diameter (DM) and/or dimple depth     (DP), and have an average depth of at least about 0.18 mm and a     diameter-to-depth ratio (DM/DP) of not more than about 23; and the     ball has a coefficient of lift CL at a Reynolds number of 70,000 and     a spin rate of 2,000 rpm which is maintained at 60% or more of the     coefficient of lift CL at a Reynolds number of 80,000 and a spin     rate of 2,000. -   [2] The multi-piece solid golf ball of [1] wherein, letting Da     represent a dimple having a diameter of at least 3.7 mm and Db     represent a dimple having a diameter of less than 3.7 mm, the ratio     (total number of Db)/(total number of Da) is at least about 0.005     but not more than about 1. -   [3] The multi-piece solid golf ball of [2], wherein dimples Da of at     least 3.7 mm account for at least about 75% of the total dimple     volume. -   [4] The multi-piece solid golf ball of [1], wherein the material     hardness (Shore D) of the inner cover layer is from 45 to 65. -   [5] The multi-piece solid golf ball of [1], wherein the material     hardness (Shore D) of the outer cover layer is from 45 to 65. -   [6] The multi-piece solid golf ball of [1], wherein the value     obtained by subtracting the material hardness (Shore D) of the outer     cover layer from the material hardness (Shore D) of the inner cover     layer is within ±4.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing the internal structure of a multi-piece solid golf ball according to an embodiment of the present invention.

FIG. 2 is a schematic view illustrating a dimple used in the present invention.

FIG. 3 is a top view of a golf ball showing a dimple pattern (I) used on a ball in an example of the invention.

FIG. 4 is a top view of a golf ball showing a dimple pattern (II) used on a ball in an example of the invention.

FIG. 5 is a front view of a golf ball showing a dimple pattern (III) used on a ball in a comparative example.

FIG. 6 is a front view of a golf ball showing a dimple pattern (IV) used on a ball in a comparative example.

FIG. 7 is a front view of a golf ball showing a dimple pattern (V) used on a ball in a comparative example.

FIG. 8 is a front view of a golf ball showing a dimple pattern (VI) used on a ball in a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention is a multi-piece solid golf ball having a solid core (referred to below as simply the “core”), an inner cover layer and an outer cover layer. The outer cover layer has a surface with a plurality of dimples formed thereon. By combining an inner cover layer and an outer cover layer having a specific relationship between the material hardnesses thereof with dimples which satisfy the subsequently described specific parameters, the distance traveled by the ball on shots at a high HS can be substantially reduced while suppressing a decrease in the distance traveled by the ball on shots at a low HS. As used in the present invention, “high HS range” refers to a range of about 50 to 60 m/s and “low HS range” refers to a range of 30 to 40 m/s.

Regarding the interior structure of the golf ball G of the present invention, as shown in FIG. 1, the ball G has a three-layer construction composed of at least a core 1, an inner cover layer 2 encasing the core 1, and an outer cover layer 3 encasing the inner cover layer 2. In this invention, the inner cover layer 2 and the outer cover layer 3 are sometimes referred to collectively as the “cover.” A plurality of dimples D are generally formed on the surface of the outer cover layer 3, and these dimples D satisfy the specific parameters of the invention. It should be noted that, although FIG. 1 shows a three-layer construction arrived at by forming a core 1, an inner cover layer 2 and an outer cover layer 3, any of these layers may optionally be formed as a plurality of two or more layers without departing from the scope of the invention. For example, the core may be formed as a plurality of layers.

The core in the invention may be formed using a rubber composition containing, for example, a base rubber and also such ingredients as a co-crosslinking agent, an organic peroxide, an inert filler, sulfur and an organosulfur compound. The base rubber of the rubber composition is preferably one composed primarily of a known polybutadiene.

In the present invention, an organosulfur compound may be optionally included in the base rubber in order to increase the rebound of the core. When an organosulfur compound is included, the amount included per 100 parts by weight of the base rubber may be set to preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight. The upper limit in the amount included may be set to preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight by weight, and even more preferably not more than 2 parts by weight. If the amount of organosulfur compound included is too small, a sufficient core rebound-increasing effect may not be obtained. On the other hand, if too much organosulfur compound is included, the core may become too soft, resulting in a poor feel when the ball is played and a poor durability to cracking on repeated impact.

The diameter of the core, although not subject to any particular limitation, may be set to from 30 to 40 mm. In this case, the lower limit value is preferably at least 32 mm, more preferably at least 34 mm, and even more preferably at least 35 mm. The upper limit value may be set to preferably not more than 39.5 mm, more preferably not more than 39 mm, and even more preferably not more than 38.5 mm.

The core deflection, meaning the amount of deflection when compressed under a final load of 1,275 N (130 k_(g)f) from an initial load of 98 N (10 kgf), although not subject to any particular limitation, may be set within a range of from 2.0 to 6.0 mm. In this case, the lower limit value is preferably at least 2.5 mm, more preferably at least 2.8 mm, and even more preferably at least 3.2 mm. The upper limit value may be set to preferably not more than 5.5 mm, more preferably not more than 5.0 mm, and even more preferably not more than 4.5 mm. If the core is too much harder than the above range (small deflection), the spin will rise excessively, which is unsuitable for the dimples of the present invention. On the other hand, if the core is too much softer than the above range (large deflection), the feel of the ball may become too soft and the durability to cracking on repeated impact may worsen.

The specific gravity of the core, while not subject to any particular limitation, may be set within a range of from 0.9 to 1.4. In such a case, the lower limit value is preferably at least 1.0, and more preferably at least 1.1. The upper limit value may be set to preferably not more than 1.3, and more preferably not more than 1.2.

In the present invention, by using the above material to form the solid core 1, the rebound can be increased, thus enabling a golf ball capable of achieving a stable trajectory to be provided.

In the golf ball G of the invention, an inner cover layer 2 and an outer cover layer 3 are formed around the above solid core 1. In this invention, it is critical for the difference between the material hardness (Shore D) of the inner cover layer and the material hardness (Shore D) of the outer cover layer to be adjusted so as to be small. Specifically, the value obtained by subtracting the material hardness (Shore D) of the outer cover layer from the material hardness (Shore D) of the inner cover layer must be greater than −5 and less than +5. It is recommended that this value be more preferably within ±4, even more preferably within ±3, and most preferably within ±2. When the above hardness difference is too large, the feel on impact may not be good. Here, “material hardness (Shore D)” refers to the hardness of a sheet of the cover material that has been formed under applied pressure to a thickness of about 2 mm, as measured using a type D durometer in general accordance with ASTM D2240. This is referred to below as the “Shore D hardness.”

The respective Shore D hardnesses and thicknesses of the inner cover layer and the outer cover layer are not subject to any particular limitation, although it is recommended that these values be set as follows in the present invention.

First of all, the Shore D hardness of the inner cover layer, although not subject to any particular limitation, may be set to at least 45, preferably at least 48, more preferably at least 51, and most preferably at least 55. It is recommended that the upper limit be not more than 65, preferably not more than 63, more preferably not more than 61, and most preferably not more than 60. When the Shore D hardness of the inner cover layer is too high, the ball may have a poor feel on impact.

The thickness of the inner cover layer, although not subject to any particular limitation, may be set to at least 0.5 mm, preferably at least 0.7 mm, more preferably at least 1.0 mm, and still more preferably at least 1.3 mm. It is recommended that the upper limit be not more than 3.0 mm, preferably not more than 2.5 mm, even more preferably not more than 2.3 mm, and most preferably not more than 2.2 mm. When the inner cover layer is too thin, the durability may worsen; when it is too thick, the ball may have a poor feel on impact.

The Shore D hardness of the outer cover layer, although not subject to any particular limitation, may be set to at least 45, preferably at least 48, more preferably at least 51, and most preferably at least 54. It is recommended that the upper limit be not more than 65, preferably not more than 63, more preferably not more than 61, and most preferably not more than 60. If the Shore D hardness of the outer cover layer is too low, the feel on impact may be too soft. On the other hand, if it is too high, the durability or the feel on impact may worsen.

The thickness of the outer cover layer, although not subject to any particular limitation, may be set to at least 0.5 mm, preferably at least 0.7 mm, and more preferably at least 0.8 mm. It is recommended that the upper limit be not more than 3.0 mm, preferably not more than 2.5 mm, more preferably not more than 2.0 mm, and even preferably not more than 1.6 mm. If the thickness of the outer cover layer falls outside the above range, this may lead to a worsening in the feel of the ball on impact or in the durability.

The cover having the above construction may be formed of a known material exemplified by thermoplastic resins such as ionomeric resins, and various types of thermoplastic elastomers. Examples of thermoplastic elastomers include polyester-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

In the present invention, such cover materials are not subject to any particular limitation, although preferred use may be made of a cover material composed primarily of a material selected from the group consisting of the polyurethane materials (I), polyurethane materials (II) and ionomeric resin materials shown below. These materials, including molding methods, are described in order below.

Polyurethane Material (I)

This material (I) is composed primarily of components A and B below:

-   (A) a thermoplastic polyurethane material, -   (B) an isocyanate mixture obtained by dispersing (B-1) an isocyanate     compound having as functional groups at least two isocyanate groups     per molecule in (B-2) a thermoplastic resin that is substantially     non-reactive with isocyanate.

Golf balls in which the cover has been formed of this material (I) can be endowed with an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact.

Next, each of the above components is described.

The thermoplastic polyurethane material (A) has a structure which includes soft segments made of a polymeric polyol (polymeric glycol), and hard segments made of a chain extender and a diisocyanate. Here, the polymeric polyol used as a starting material is not subject to any particular limitation, and may be any that is used in the prior art relating to thermoplastic polyurethane materials, such as polyester polyols and polyether polyols. Polyether polyols are preferable to polyester polyols because they enable the synthesis of thermoplastic polyurethane materials having a high rebound resilience and excellent low-temperature properties. Illustrative examples of polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred from the standpoint of the rebound resilience and low-temperature properties. The polymeric polyol has an average molecular weight of preferably from 1,000 to 5,000. A molecular weight of from 2,000 to 4,000 is especially preferred for synthesizing thermoplastic polyurethane materials having a high rebound resilience.

The chain extender employed is preferably one which is used in the art relating to conventional thermoplastic polyurethane materials. Illustrative, non-limiting, examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which is used in the art relating to conventional thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, for stable reactivity with the subsequently described isocyanate mixture (B), it is most preferable to use the following aromatic diisocyanate: 4,4′-diphenylmethane diisocyanate.

A commercial product may be advantageously used as the thermoplastic polyurethane material composed of the above-described material. Illustrative examples include those available under the trade names Pandex T-8290, Pandex T-8295 and Pandex T8260 (DIC Bayer Polymer, Ltd.), and those available under the trade names Resamine 2593 and Resamine 2597 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

The isocyanate mixture (B) is obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate. Here, the isocyanate compound (B-1) is preferably an isocyanate compound used in the prior art relating to thermoplastic polyurethane materials. Illustrative, non-limiting, examples include aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. From the standpoint of reactivity and work safety, the use of 4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (B-2) is preferably a resin having a low water absorption and excellent compatibility with thermoplastic polyurethane materials. Illustrative examples of such resins include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins, and polyester elastomers (e.g., polyether-ester block copolymers, polyester-ester block copolymers). From the standpoint of the rebound resilience and strength, the use of a polyester elastomer, particularly a polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relative proportions of the thermoplastic resin (B-2) and the isocyanate compound (B-1), expressed as the weight ratio (B-2):(B-1), to be from 100:5 to 100:100, and especially from 100:10 to 100:40. If the amount of the isocyanate compound (B-1) relative to the thermoplastic resin (B-2) is too small, a greater amount of the isocyanate mixture (B) will have to be added to achieve an amount of addition sufficient for the crosslinking reaction with the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (B-2) will exert a large influence, rendering the physical properties of the material inadequate. On the other hand, if the amount of the isocyanate compound (B-1) relative to the thermoplastic resin (B-2) is too large, the isocyanate compound (B-1) may cause slippage to occur during mixing, making preparation of the isocyanate mixture (B) difficult.

The isocyanate mixture (B) may be obtained by, for example, adding the isocyanate compound (B-1) to the thermoplastic resin (B-2) and thoroughly working together these components at a temperature of from 130 to 250° C. using mixing rolls or a Banbury mixer, then either pelletizing or cooling and subsequently grinding. A commercial product such as that available under the trade name Crossnate EM30 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as the isocyanate mixture (B).

The above material (I) is composed primarily of the thermoplastic polyurethane material (A) and isocyanate mixture (B) described above. In this material (I), the isocyanate mixture (B) is included in an amount, per 100 parts by weight of the thermoplastic polyurethane material (A), of at least 1 part by weight, preferably at least 5 parts by weight, and more preferably at least 10 parts by weight, but not more than 100 parts by weight, preferably not more than 50 parts by weight, and more preferably not more than 30 parts by weight. If too little isocyanate mixture (B) is included relative to the thermoplastic polyurethane material (A), a sufficient crosslinking effect will not be achieved. On the other hand, if too much is included, this may lead to discoloration of the molded material by unreacted isocyanate, which is undesirable.

In addition to above components (A) and (B), another component (C), although not essential, may also be included in the material (I). This other component is exemplified by thermoplastic polymeric materials other than thermoplastic polyurethane materials; illustrative examples include polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, polyethylene, and nylon resins. When component (C) is included, the amount is not subject to any particular limitation and may be suitably selected as appropriate for such purposes as adjusting the hardness, improving the resilience, improving the flow properties, and improving the adhesion of the cover material. The amount of component (C) included per 100 parts by weight of component (A) is set to preferably at least 10 parts by weight, and the upper limit is set to not more than 100 parts by weight, preferably not more than 75 parts by weight, and more preferably not more than 50 parts by weight. If necessary, various additives such as pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers and parting agents may also be suitably included in the above material (I).

Formation of the cover using the above material (I) may be carried out by a known molding method. For example, the cover may be molded by adding the isocyanate mixture (B) to the thermoplastic polyurethane material (A) and dry mixing, feeding the resulting mixture to an injection molding machine, and injecting the molten resin blend over the core. In such a case, the molding temperature varies with the type of thermoplastic polyurethane material (A), although molding is generally carried out within a temperature range of 150 to 250° C.

Reactions and crosslinking which take place in the golf ball cover obtained as described above are believed to involve the reaction of isocyanate groups with hydroxyl groups remaining in the thermoplastic polyurethane material to form urethane bonds, or the creation of an allophanate or biuret crosslinked form via a reaction involving the addition of isocyanate groups to urethane groups in the thermoplastic polyurethane material. Although the crosslinking reaction has not yet proceeded to a sufficient degree immediately after injection molding of the material (I), the crosslinking reaction can be made to proceed further by carrying out an annealing step after molding, in this way conferring the golf ball cover with useful characteristics. “Annealing,” as used herein, refers to heat aging the cover at a constant temperature for a fixed length of time, or aging the cover for a fixed period at room temperature.

Polyurethane Material (II)

This material (II) is a single resin blend in which the primary components are (D) a thermoplastic polyurethane and (E) a polyisocyanate compound. By forming a cover composed primarily of such a polyurethane material (II), it is possible to achieve an excellent feel, controllability, cut resistance, scuff resistance and durability to cracking on repeated impact without a loss of resilience.

As used herein, reference to a “single” resin blend means that the resin blend is not fed as a plurality of types of pellets, but rather is supplied to, for example, an injection molding machine as one type of pellet prepared by incorporating a plurality of ingredients into individual pellets.

To fully and effectively achieve the objects of the invention, a necessary and sufficient amount of unreacted isocyanate groups should be present within the cover resin material. Specifically, it is recommended that the combined weight of above components (D) and (E) account for at least 60%, and preferably at least 70%, of the total weight of the cover. Components (D) and (E) are described in detail below.

The above thermoplastic polyurethane (D) is described. The thermoplastic polyurethane structure includes soft segments made of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments made of a chain extender and a polyisocyanate compound. Here, the long-chain polyol used as a starting material is not subject to any particular limitation, and may be any that has hitherto been used in the art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof. Of the long-chain polyols mentioned here, polyether polyols are preferred because they enable the synthesis of thermoplastic polyurethanes having a high rebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of cyclic ethers. These polyether polyols may be used singly or as a combination of two or more thereof. In the present invention, poly(tetramethylene glycol) and poly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-average molecular weight in a range of 1,500 to 5,000. By using a long-chain polyol having a number-average molecular weight within this range, golf balls made with a thermoplastic polyurethane composition having excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in a range of 1,700 to 4,000, and even more preferably in a range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chain polyol” refers to the number-average molecular weight calculated based on the hydroxyl number measured in accordance with JIS K-1557.

Any chain extender employed in the prior art relating to thermoplastic polyurethane materials may be advantageously used as the chain extender. For example, low-molecular-weight compounds with a molecular weight of 400 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. In the present invention, an aliphatic diol having 2 to 12 carbons is preferred, and 1,4-butylene glycol is more preferred.

Any polyisocyanate compound employed in the prior art relating to thermoplastic polyurethane materials may be advantageously used without particular limitation as the polyisocyanate compound. For example, use may be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. However, depending on the type of isocyanate, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving as above component D to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be a polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the aromatic diisocyanate to be 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction can be adjusted within a desirable range so as to make it possible to obtain a golf ball which is composed of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing component (D). Production may be carried out by either a prepolymer process or a one-shot process in which the long-chain polyol, chain extender and polyisocyanate compound are used and a known urethane-forming reaction is effected. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

A commercial product may be used as component (D). Illustrative examples include products available under the trade names Pandex T8295, Pandex T8290 and Pandex T8260 (DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component E, it is essential that, in at least some portion thereof within a single resin blend, all the isocyanate groups on the molecule remain in an unreacted state. That is, polyisocyanate compound in which all the isocyanate groups on the molecule are in a completely free state should be present within a single resin blend, and such a polyisocyanate compound may be present together with a polyisocyanate compound in which a portion of the isocyanate groups on the molecule are in a free state.

Various isocyanates may be used without particular limitation as the polyisocyanate compound. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, using 4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferred for achieving a good balance between the influence on moldability by, for example, the rise in viscosity associated with reaction with the thermoplastic polyurethane serving as component D, and the properties of the resulting golf ball cover material.

In material (II), although not an essential ingredient, a thermoplastic elastomer other than the above thermoplastic polyurethane may be included as component F in addition to above components D and E. Including this component F in the above resin blend enables the flow properties of the resin blend to be further improved and enables various properties required of golf ball cover materials, such as resilience and scuff resistance, to be enhanced.

This component F, which is a thermoplastic elastomer other than the above thermoplastic polyurethane, is exemplified by one or more thermoplastic elastomer selected from among polyester elastomers, polyamide elastomers, ionomeric resins, styrene block elastomers, hydrogenated styrene-butadiene rubbers, styrene-ethylene/butylene-ethylene block copolymers and modified forms thereof, ethylene-ethylene/butylene-ethylene block copolymers and modified forms thereof, styrene-ethylene/butylene-styrene block copolymers and modified forms thereof, ABS resins, polyacetals, polyethylenes and nylon resins. The use of polyester elastomers, polyamide elastomers and polyacetals is especially preferred because the resilience and scuff resistance are enhanced, owing to reactions with isocyanate groups, while at the same time a good manufacturability is retained.

The relative proportions of above components D, E and F are not subject to any particular limitation. However, to fully achieve the advantageous effects of the invention, it is preferable for the weight ratio among the respective components to be (D):(E):(F) =100:2 to 50:0 to 50, and more preferably (D):(E):(F) =100:2 to 30:8 to 50.

In this invention, a single resin blend for forming the cover is prepared by mixing together component D, component E, and also optional component F. At this time, it is essential to select the mixing conditions such that, of the polyisocyanate compound, at least some polyisocyanate compound is present in which all the isocyanate groups on the molecule remain in an unreacted state. For example, treatment such as mixture in an inert gas (e.g., nitrogen) or in a vacuum state must be furnished. The resin blend is then injection-molded around a core which has been placed in a mold. To smoothly and easily handle the resin blend, it is preferable for the blend to be formed into pellets having a length of 1 to 10 mm and a diameter of 0.5 to 5 mm. Sufficient isocyanate groups in an unreacted state remain in these resin pellets; the unreacted isocyanate groups react with component D or component F to form a crosslinked material while the resin blend is being injection-molded about the core, or due to post-treatment such as annealing thereafter.

In addition, various optional additives may also be included in this cover-forming resin blend. For example, pigments, dispersants, antioxidants, light stabilizers, ultraviolet absorbers, and parting agents may be suitably included.

The melt mass flow rate (MFR) of this resin blend at 210° C. is not subject to any particular limitation. However, to increase the flow properties and manufacturability, the MFR is preferably at least 5 g/10 min, and more preferably at least 6 g/10 min. If the melt mass flow rate of the resin blend is too low, the flow properties will decrease, which may cause eccentricity during injection molding and may also lower the degree of freedom in the thickness of the cover that can be molded. The melt mass flow rate is a measured value obtained in accordance with JIS-K7210 (1999 edition).

The method of molding the cover may involve feeding the above resin blend to an injection-molding machine and injecting the molten resin blend around the core. Although the molding temperature in this case will vary depending on the type of thermoplastic polyurethane, the molding temperature is generally from 150 to 250° C.

When injection molding is carried out, it is desirable though not essential to carry out molding in a low-humidity environment such as by purging with an inert gas (e.g., nitrogen) or a low-moisture gas (e.g., low dew-point dry air), or vacuum treating, some or all places on the resin paths from the resin feed area to the mold interior. Illustrative, non-limiting, examples of the medium used for transporting the resin include low-moisture gases such as low dew-point dry air or nitrogen. By carrying out molding in such a low-humidity environment, reaction by the isocyanate groups is kept from proceeding before the resin has been charged into the mold interior. As a result, polyisocyanate in which the isocyanate groups are present in an unreacted state is included to some degree in the molded resin material, thus making it possible to reduce variable factors such as an unnecessary in viscosity and enabling the real crosslinking efficiency to be enhanced.

Techniques that may be used to confirm the presence of polyisocyanate compound in an unreacted state within the resin blend prior to injection molding about the core include those which involve extraction with a suitable solvent that selectively dissolves out only the polyisocyanate compound. An example of a simple and convenient method is one in which confirmation is carried out by simultaneous thermogravimetric and differential thermal analysis (TG-DTA) measurement in an inert atmosphere. For example, when the above-described single resin blend (material (II)) is heated in a nitrogen atmosphere at a temperature ramp-up rate of 10° C./min, a gradual drop in the weight of diphenylmethane diisocyanate can be observed from about 150° C. On the other hand, in a resin sample in which the reaction between the thermoplastic polyurethane material and the isocyanate mixture has been carried out to completion, a weight drop is not observed from about 150° C., but a weight drop can be observed from about 230 to 240° C.

After the above material (II) has been injection-molded to form a cover, the properties as a golf ball cover can be additionally improved by carrying out annealing so as to induce the crosslinking reaction to proceed further. “Annealing,” as used herein, refers to aging the cover in a fixed environment for a fixed length of time.

Ionomeric Resin Material

In the present invention, “ionomeric resin material” refers to a resin composition which includes: 100 parts by weight of a resin component composed of

a base resin containing (a) from 95 to 50 wt % of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer and/or a metal salt thereof and (b) from 0 to 20 wt % of an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof, and

(c) from 0 to 50 wt % of a thermoplastic block copolymer composed of a crystalline polyolefin block and a polyethylene/butylene random copolymer;

(d) from 5 to 170 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of 280 to 1,500; and

(e) from 0.1 to 10 parts by weight of a basic inorganic metal compound capable of neutralizing acid groups in components (a) and (d), and, if necessary, component (b).

Components (a) to (e) are described below.

Component (a) and component (b) serve as the base resin of the above resin composition. Component (a) is an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymer and/or metal salt thereof, and component (b) is an olefin-unsaturated carboxylic acid random copolymer and/or a metal salt thereof. In the present invention, either of above components (a) and (b) may be used singly or both may used in combination.

Here, above component (a) has a weight-average molecular weight (Mw) of preferably at least 100,000, more preferably at least 110,000, and even more preferably at least 120,000, but preferably not more than 200,000, more preferably not more than 190,000, and even more preferably not more than 170,000. The weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio for the copolymer is preferably at least 3, and more preferably at least 4, with the upper limit being preferably not more than 7, and more preferably not more than 6.5.

The olefin in component (a) generally has a number of carbons that is at least 2, but not more than 8, and preferably not more than 6. Illustrative examples of such olefins include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester may be, for example, a lower alkyl ester of an unsaturated carboxylic acid. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, isobutyl acrylate) is especially preferred.

The random copolymer serving as component (a) may be obtained by the random copolymerization of the above ingredients in accordance with a known method. Here, the unsaturated carboxylic acid content (acid content) within the random copolymer, although not subject to any particular limitation, may be set to generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %. It is recommended that the upper limit in the unsaturated carboxylic acid content (acid content), although not subject to any particular limitation, be generally not more than 25 wt %, preferably not more than 20 wt %, and more preferably not more than 15 wt %. At a low acid content, the rebound may decrease, whereas at a high acid content, the processability of the material may decrease.

The copolymer of component (a) accounts for a proportion of the overall base resin which is preferably from 95 to 50 wt %. The lower limit of this proportion is preferably at least 60 wt %, more preferably at least 70 wt %, and even more preferably at least 75 wt %. The upper limit to is preferably not more than 92 wt %, more preferably not more than 89 wt %, and most preferably not more than 86 wt %.

The metal salt of the copolymer of component (a) may be obtained by neutralizing some of the acid groups in the random copolymer of component (a) with metal ions. Here, the metal ions which neutralize the acid groups are exemplified by Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In the present invention, of these, preferred use may be of Na⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ in particular, and Zn⁺⁺ is even more recommended. The degree of neutralization of the random copolymer by these metal ions, while not subject to any particular limitation, is generally at least 5 mol %, preferably at least 10 mol %, and especially at least 20 mol %. It is recommended that the upper limit in the degree of neutralization, while not subject to any particular limitation, be generally not more than 95 mol %, preferably not more than 90 mol %, and especially not more than 80 mol %. At a degree of neutralization in excess of 95 mol %, the moldability may decrease. On the other hand, at less than 5 mol %, it is necessary to increase the amount in which the inorganic metal compound serving as component (c) is added, which may present a drawback in terms of cost. Such a neutralization product may be obtained by a known method. For example, the neutralization product may be obtained by introducing a metal ion compound, such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide, into the random copolymer.

A commercial product may be used as component (a). Illustrative examples of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymers include those available under the trade names Nucrel AN4318, Nucrel AN4319, and Nucrel AN4311 (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of metal salts of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random copolymers include those available under the trade names Himilan AM7316, Himilan AM7331, Himilan 1855 and Himilan 1856 (DuPont-Mitsui Polychemicals Co., Ltd.), and those available under the trade names Surlyn 6320 and Surlyn 8120 (E.I. DuPont de Nemours and Co., Ltd.).

Next, it is recommended that the weight-average molecular weight (Mw) of component (b) be preferably at least 100,000, more preferably at least 110,000, and even more preferably at least 120,000, and that the upper limit thereof be preferably not more than 200,000, more preferably not more than 190,000, and even more preferably not more than 170,000. The weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio for the copolymer is preferably at least 3, and more preferably at least 4, and the upper limit thereof is preferably not more than 7, and more preferably not more than 6.5.

Here, the olefin in component (b) is generally an olefin in which the number of carbons is at least 2 but not more than 8, and preferably not more than 6. Illustrative examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. The use of ethylene is especially preferred.

Illustrative examples of the unsaturated carboxylic acid in component (b) include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

In addition, the random copolymer serving as component (b) may be obtained by the random copolymerization of the above ingredients in accordance with a known method. Here, the unsaturated carboxylic acid content (acid content) within the random copolymer, while not subject to any particular limitation, may be set to generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %. No particular limitation is imposed on the upper limit in the unsaturated carboxylic acid content (acid content), although it is recommended that this be generally not more than 25 wt %, preferably not more than 20 wt %, and more preferably not more than 15 wt %. At a low acid content, there is a possibility that the rebound will decrease, whereas at a high acid content, there is a possibility that the material processability will decrease.

In the above case, the copolymer of component (b) accounts for a proportion of the overall base resin which may be set to more than 0, and may be set to preferably at least 1 wt %. The upper limit, although not subject to any particular limitation, may be set to not more than 20 wt %, preferably not more than 17 wt %, more preferably not more than 10 wt %, even more preferably not more than 8 wt %, and most preferably not more than 5 wt %.

The metal salt of the copolymer of component (b) may be obtained by neutralizing some of the acid groups in the random copolymer of component (b) with metal ions. Here, preferred use may be made of, for example, Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ or Pb⁺⁺, as the metal ions which neutralize the acid groups. In the present invention, of these, more preferred use may be made of Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ or Ca⁺⁺. The use of Zn⁺⁺ is especially recommended. The degree of neutralization of the random copolymer by these metal ions, while not subject to any particular limitation, may be set to generally at least 5 mol %, preferably at least 10 mol %, and especially at least 20 mol %. It is recommended that the upper limit in the degree of neutralization, while not subject to any particular limitation, be set to generally not more than 95 mol %, preferably not more than 90 mol %, and especially not more than 80 mol %. At a degree of neutralization in excess of 95 mol %, the moldability may decrease. On the other hand, at less than 5 mol %, there arises a need to increase the amount in which the inorganic metal compound serving as component (c) is added, which may present a drawback in terms of cost. Such a neutralization product may be obtained by a known method. For example, the neutralization product may be obtained by introducing a metal ion compound, such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide, into the random copolymer.

A commercial product may be used as component (b). Illustrative examples include those available under the trade names Nucrel 1560, Nucrel 1525 and Nucrel 1035 (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of metal salts of the olefin-unsaturated carboxylic acid random copolymer include those available under the trade names Himilan 1605, Himilan 1601, Himilan 1557, Himilan 1705 and Himilan 1706 (DuPont-Mitsui Polychemicals Co., Ltd.), those available under the trade names Surlyn 7930 and Surlyn 7920 (E.I. DuPont de Nemours and Co., Ltd.), and those available under the trade names Escor 5100 and Escor 5200 (ExxonMobil Chemical).

Component (c) is a thermoplastic block copolymer composed of a crystalline polyolefin block and a polyethylene/butylene random copolymer. This component (c) is exemplified by thermoplastic block copolymers composed of a crystalline polyethylene block (E) as a hard segment and a block of a relatively random copolymer of ethylene and butylene (EB) as a soft segment. Preferred use may be made of block copolymers having a molecular structure with a hard segment at one or both ends, such as block copolymers having an E-EB or E-EB-E structure.

Such a component (c) may be obtained by hydrogenating a polybutadiene. Here, the polybutadiene used in hydrogenation is preferably one in which bonding within the butadiene structure is characterized by a 1,4-bond content in the butadiene structure as a whole of from 95 to 100 wt %, and in which from 50 to 100 wt %, and preferably from 80 to 100 wt %, of the 1,4-bonds are present as block-like regions.

The above-mentioned E-EB-E type thermoplastic block copolymer is preferably one obtained by hydrogenating a polybutadiene having at both ends of the molecular chain 1,4-polymerization products which are rich in 1,4-bonds and having an intermediate region where 1,4-bonds and 1,2-bonds are intermingled. The degree of hydrogenation (conversion of double bonds on the polybutadiene to saturated bonds) in the polybutadiene hydrogenate is preferably from 60 to 100%, and more preferably from 90 to 100%. Too low a degree of hydrogenation may give rise to undesirable effects such as gelation in the blending step with other components such as an ionomeric resin and, when the golf ball has been formed, may lead to a poor durability to impact.

In the block copolymer having an E-EB or E-EB-E molecular structure with a hard segment at one or both ends that may be advantageously used as the thermoplastic block copolymer, the content of the hard segments is preferably from 10 to 50 wt %. If the hard segment content is too high, the cover may lack sufficient softness, making it difficult to effectively achieve the objects of the invention. On the other hand, if the hard segment content is too low, the blend may have a poor moldability.

The thermoplastic block copolymer has a melt mass flow index, at a test temperature of 230° C. and a test load of 21.2 N, of preferably from 0.01 to 15 g/10 min, and more preferably from 0.03 to 10 g/10 min. Outside of this range, problems such as weld lines, sink marks and short shots may arise during injection molding. Moreover, it is preferable for the thermoplastic block copolymer to have a surface hardness of from 10 to 50. If the surface hardness is too low, the golf ball may have a decreased durability to repeated impact. On the other hand, if the surface hardness is too high, a blend of the thermoplastic block copolymer with an ionomeric resin may have a decreased rebound. The thermoplastic block copolymer has a number-average molecular weight of preferably from 30,000 to 800,000.

A commercial product may be used as component (c). Illustrative examples include those available under the trade names Dynaron 6100P, Dynaron 6200P and Dynaron 6201B (JSR Corporation). Of these, Dynaron 6100P, which is a block polymer having crystalline olefin blocks at both ends, is especially preferred for use in the present invention. These olefinic thermoplastic elastomers may be used singly or as mixtures of two or more thereof.

In cases where component (c) is included in the resin component, the proportion of the overall resin component accounted for by component (c) may be set to more than 0, and is preferably set to at least 5 wt %, more preferably at least 8 wt %, even more preferably at least 11 wt %, and most preferably at least 14 wt %. The upper limit, while not subject to any particular limitation, may be set to preferably not more than 50 wt %, more preferably not more than 40 wt %, even more preferably not more than 30 wt %, and most preferably not more than 20 wt %.

Component (d) is a fatty acid or fatty acid derivative having a molecular weight of at least 280 but not more than 1,500 whose purpose is to enhance the flow properties of the resin composition. It has a molecular weight which is very small compared with those of components (a) to (c), and helps to significantly decrease the melt viscosity of the mixture. Also, because the fatty acid (or fatty acid derivative) of component (d) has a molecular weight of at least 280 but not more than 1,500 and has a high content of acid groups (or derivative moieties thereof), its addition results in little loss of rebound.

The fatty acid or fatty acid derivative serving as component (d) may be an unsaturated fatty acid or fatty acid derivative having a double bond or triple bond in the alkyl moiety, or it may be a saturated fatty acid or fatty acid derivative in which all the bonds in the alkyl moiety are single bonds. It is recommended that the number of carbon atoms on the molecule be generally at least 18, with an upper limit of not more than 80, and especially not more than 40. Too few carbons may make it impossible to achieve an improved heat resistance and may also set the acid group content so high as to cause the acid groups to interact with acid groups present in the base resin, diminishing the flow-improving effects. On the other hand, too many carbons increases the molecular weight, as a result of which significant flow-improving effects may not appear, which may make the material difficult to use.

Specific examples of fatty acids that may be used as component (d) include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Of these, preferred use may be made of stearic acid, arachidic acid, behenic acid, lignoceric acid and oleic acid.

Fatty acid derivatives are exemplified by derivatives in which the proton on the acid group of the fatty acid has been substituted. Exemplary fatty acid derivatives of this type include metallic soaps in which the proton has been substituted with a metal ion. Metal ions that may be used in such metallic soaps include Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used as component (d) include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred.

The amount of component (d) included per 100 parts by weight of the resin component is at least 5 parts by weight, preferably at least 20 parts by weight, more preferably at least 50 parts by weight, and even more preferably at least 85 parts by weight. The upper limit in the amount included per 100 parts by weight of the resin component is not more than 170 parts by weight, preferably not more than 150 parts by weight, more preferably not more than 130 parts by weight, and even more preferably not more than 110 parts by weight.

Use may also be made of known metallic soap-modified ionomers (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Disclosure WO 98/46671) when using above components (a) and (b).

The basic inorganic metal compound of component (e) is included so as to neutralize acid groups in above component (a), component (d) and, if necessary, component (b). When above component (d) is not included, and in particular when a metal-modified ionomeric resin alone (e.g., a metal soap-modified ionomeric resin of the type mentioned in the foregoing patent publications, alone) is heated and mixed, as mentioned below, the metallic soap and unneutralized acid groups present on the ionomer undergo exchange reactions, generating a fatty acid. Because this fatty acid has a low thermal stability and readily vaporizes during molding, it causes molding defects. Moreover, if the fatty acid thus generated deposits on the surface of the molded material, it substantially lowers paint film adhesion.

-   (1) unneutralized acid group present on the ionomeric resin -   (2) metallic soap -   (3) fatty acid -   X: metal cation

To resolve such problems, a basic inorganic metal compound which neutralizes the acid groups present in above components (a), (b) and (d) is thus included as an essential component (component (e)). By adding component (e), the acid groups in above components (a), (b) and (d) are neutralized. Synergistic effects from the inclusion of these respective components increase the thermal stability of the resin composition while at the same time conferring a good moldability, and also impart the excellent property of enhancing rebound as a golf ball material.

It is recommended that component (e) be a basic inorganic metal compound—preferably a monoxide or hydroxide—which is capable of neutralizing acid groups in above components (a), (b) and (d). Because such compounds have a high reactivity with the ionomeric resin and the reaction by-products contain no organic matter, the degree of neutralization of the resin composition can be increased without a loss of thermal stability.

The metal ions used here in the basic inorganic metal compound are exemplified by Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺Co⁺⁺. Illustrative examples of the inorganic metal compound include basic inorganic fillers containing these metal ions, such as magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. Of these, as noted above, a monoxide or hydroxide is preferred. The use of magnesium oxide or calcium hydroxide, which have high reactivities with ionomer resins, is especially preferred in the present invention.

Component (e) is included in an amount, per 100 parts by weight of the resin component, of from 0.1 to 10 parts by weight. In this case, the lower limit is preferably at least 0.5 part by weight, more preferably at least 0.8 part by weight, and even more preferably at least 1 part by weight. The upper limit in the amount included per 100 parts by weight of the resin component is not more than 8 parts by weight, preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

The above-described resin composition which is obtained by blending components (a) to (e) can be provided with improved thermal stability, moldability and resilience.

To this end, it is recommended that at least 70 mol %, preferably at least 80 mol %, and more preferably at least 90 mol %, of the acid groups in the resin composition be neutralized. A high degree of neutralization more reliably suppresses the exchange reactions that pose a problem in the above-described cases where components (a) and (b) and the fatty acid (or fatty acid derivative) alone are used, thus making it possible to prevent the generation of fatty acids. As a result, a material can be obtained which has a markedly increased thermal stability, a good moldability, and a substantially higher resilience than conventional ionomeric resins.

Here, with regard to neutralization of the above resin composition, to more reliably achieve both a high degree of neutralization and good flow properties, it is recommended that the acid groups in the resin composition be neutralized with transition metal ions and with alkali metal and/or alkaline earth metal ions. Because transition metal ions have a weaker ionic cohesion than alkali metal and alkaline earth metal ions, it is possible in this way to neutralize some of the acid groups in the resin composition and thus enable the flow properties to be significantly improved.

Various additives may also be optionally included in the above resin composition. Examples of additives which may be suitably included are pigments, dispersants, antioxidants, ultraviolet absorbers and optical stabilizers. Moreover, to further improve the feel of the golf ball on impact, the resin composition may also include various non-ionomeric thermoplastic elastomers. Illustrative examples of such non-ionomeric thermoplastic elastomers include styrene-based thermoplastic elastomers, ester-based thermoplastic elastomers and urethane-based thermoplastic elastomers. In this invention, the use of styrene-based thermoplastic elastomers is especially preferred.

Use may be made of a known mixing apparatus when preparing the above resin composition. For example, the above respective ingredients may be mixed using a twin-screw extruder, a Banbury mixer or a kneader. In such a case, the heating and mixing conditions may be suitably selected according to the type of material, and are not subject to any particular limitation. For example, mixing may be carried out at a temperature of from 150 to 250° C. The method of molding the cover using the above resin composition is also not subject to any particular limitation. For example, use may be made of an injection molding method or a compression molding method. When injection molding is employed, the process may involve placing a prefabricated core at a given position in the injection mold, then introducing the above material into the mold. When compression molding is employed, the process may involve producing a pair of half cups from the above material, covering the core with these half-cups, then applying pressure and heat within a mold. If molding under heat and pressure is carried out, the molding conditions used may be a temperature of from 120 to 170° C. and a period of from 1 to 5 minutes.

The cover material used in the invention may be a known cover material. Although not subject to any particular limitation, preferred use may be made of the above-described polyurethane material (I), polyurethane material (II) or an ionomeric resin material.

In the inventive golf ball, by combining dimples which satisfy the subsequently described specific parameters and are able to achieve a relatively low trajectory with an inner cover layer and an outer cover layer having the above-described specific constructions, it is possible to greatly reduce the distance traveled by the golf ball on shots taken at a high head speed and also hold down the decrease in distance traveled on shots taken at a low head speed. The parameters for the dimples formed in the inventive golf ball are described in detail below.

In the present invention, dimples having the following parameters (1) to (8) are formed on the surface of the cover made of the above-described material. In cases where the surface of the ball is subjected to finishing treatment (e.g., painting and stamping) after the cover has been formed, parameters (1) to (8) below are calculated based on the shape of the dimples on the finished ball in which such treatment has been fully completed.

Dimple Parameter (1)

The total number of dimples is set in a range of at least 250 but not more than 500. In this case, the lower limit may be set to preferably at least 280, more preferably at least 300, and even more preferably at least 340. The upper limit may be set to preferably not more than 450, more preferably not more than 420, and even more preferably not more than 400. In this range, the golf ball readily incurs lift, enabling the ball to travel farther, particularly on shots with a driver.

Dimple Parameter (2)

To improve aerodynamic performance, the dimple surface coverage (SR), defined as the sum of the surface areas on a hypothetical sphere that are circumscribed by the edges of the respective dimples as a proportion of the surface area of the hypothetical sphere, is set to at least 70%. SR may be set to preferably at least 71%, and more preferably at least 72%.

Dimple Parameter (3)

To improve the aerodynamic performance, the dimple volume ratio (VR), defined as the sum of the volumes of individual dimple spaces below a flat plane circumscribed by the edge of each dimple on a golf ball as a proportion of the volume of the golf ball were it to have no dimples on the surface (hypothetical sphere), is set to at least 1.0%. It is recommended that VR be set to preferably at least 1.1%, more preferably at least 1.15%, and even more preferably at least 1.2%. The upper limit is not more than 1.5%, preferably not more than 1.4%, and more preferably not more than 1.3%.

Dimple Parameter (4)

The number of dimple types, i.e., types of dimples of mutually differing diameter DM and/or depth DP, is set to three or more. Here, the number of types is set to preferably at least four, and more preferably at least five. The upper limit is not more than 14 types, and preferably not more than 10 types. The number of types of dimples is selected as appropriate in this way so as to facilitate an to increase in the surface coverage SR specified in the invention.

Here, referring to FIG. 2, the depth DP of a dimple is the vertical distance from a hypothetical flat plane L, traced by connecting the positions where the dimple meets land areas, to the bottom (deepest position) of the dimple. The dimple diameter DM, as shown in FIG. 2, is the diameter (span) between positions where the dimple portion is tangent with land areas (non-dimple forming portions), i.e., between the high points of the dimple portion. In most cases, the golf ball has been subjected to painting or the like. In such balls, the dimple diameter and depth refer to the diameter and depth after the coat of paint has been applied.

Dimple Parameter (5)

To obtain a proper trajectory, the average dimple depth is set to at least about 0.18 mm. It is recommended that the average dimple depth be set to preferably at least about 0.19. The upper limit is not more than about 1.0 mm, preferably not more than about 0.7 mm, and more preferably not more than about 0.5 mm. Here, “average dimple depth” refers to the average of the depths of all the dimples.

The average dimple diameter DM, while not subject to any particular limitation, is preferably at least about 3.0 mm, more preferably at least about 3.2 mm, and even more preferably at least about 3.5 mm. The upper limit is preferably not more than about 7.5 mm, more preferably not more than about 6.5 mm, and even more preferably not more than about 6 mm. Here, “average dimple diameter DM” refers to the average of the diameters of all the dimples.

Dimple Parameter (6)

The ratio of the dimple diameter DM to the dimple depth DP, or DM/DP, has an average value of not more than about 23. It is recommended that this average value be preferably not more than about 22, more preferably not more than about 21, and even more preferably not more than about 20. The lower limit, while not subject to any particular limitation, is preferably at least about 5, more preferably at least about 8, and even more preferably at least about 10.

Dimple Parameter (7)

In the present invention, although not subject to any particular limitation, when the dimples are divided into dimples Da having a diameter of 3.7 mm or more and smaller dimples Db, the (total number of Da)/(total number of Db) ratio is preferably set to at least about 0.005 but not more than about 1. The lower limit is more preferably at least about 0.01, even more preferably at least about 0.1, still more preferably at least about 0.2, and most preferably at least about 0.3. The upper limit is more preferably not more than about 0.8, even more preferably not more than about 0.6, and most preferably not more than about 0.5.

The dimples Da having a diameter of at least 3.7 mm account for a proportion of the total dimple volume which, while not subject to any particular limitation, is preferably at least about 75%, more preferably at least about 78%, and even more preferably at least about 80%. The upper limit value is preferably not more than about 98%, more preferably not more than about 95%, and even more preferably not more than about 92%.

The average diameter (Dm) of the Da dimples is preferably at least about 3.7 mm, and more preferably at least about 3.8 mm. The upper limit thereof is preferably not more than about 7 mm, and more preferably not more than about 6 mm. The average depth (Dp) of the Da dimples is preferably at least about 0.05 mm, and more preferably at least about 0.1 mm. The upper limit thereof is preferably not more than about 0.5 mm, and more preferably not more than about 0.3 mm. The average volume of the Da dimples is preferably at least about 0.8 mm³, and more preferably at least about 1.0 mm³. The upper limit thereof is preferably not more than about 3.0 mm³, and more preferably not more than about 2.5 mm³. The ratio Dm/Dp for the Da dimples is preferably at least about 7, and more preferably at least about 8, and the upper limit thereof is preferably not more than about 25, and more preferably not more than about 23. If the above numerical value ranges are not satisfied, the low trajectory desired may not be obtained, which may make it impossible to achieve the objects of the invention.

The average diameter (Dm) of the Db dimples is preferably at least about 1 mm, and more preferably at least about 2 mm. The upper limit is preferably less than about 3.7 mm, and more preferably not more than about 3.5 mm. The average depth (Dp) of the Db dimples is preferably at least about 0.05 mm, and more preferably at least about 0.1 mm. The upper limit thereof is preferably not more than about 0.3 mm, and more preferably not more than about 0.2 mm. The average volume of the Db dimples is preferably at least about 0.2 mm³, and more preferably at least about 0.3 mm³. The upper limit thereof is preferably not more than about 1.5 mm³, and more preferably not more than about 1.0 mm³. The ratio Dm/Dp for the Db dimples is preferably at least about 10, and more preferably at least about 12. The upper limit thereof is preferably not more than about 30, and more preferably not more than about 26. If the above numerical value ranges are not satisfied, the low trajectory desired may not be obtained, which may make it impossible to achieve the objects of the invention.

Dimple Parameter (8)

To improve the distance a golf ball travels, it is desirable for the ball to have a low coefficient of drag (CD) under high-velocity conditions and a high coefficient of lift (CL) under low-velocity conditions. Thus, in the present invention, with regard to the low-velocity CL, it is critical for the coefficient of lift CL when the ball is launched using an Ultra Ball Launcher (UBL) at a Reynolds number of 70,000 and a spin rate of 2,000 rpm to be maintained at 60% or more, and preferably at 65% or more, of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm.

The shapes of the dimples are not limited to a circular shape, and may also be suitably selected from among, for example, polygonal, tear-shaped and oval shapes. Setting the number of dimple types to at least three, and preferably at least five, makes it possible for the dimples to cover a higher proportion of the spherical surface. Also, by interspersing large and small dimples, the surface coverage can be increased to the specified range. Because this enables extreme fluctuations in the CL (coefficient of lift) within the low-velocity region to be suppressed, the ball trajectory can be made relatively low, thus making it easier to elicit the advantageous effects of the invention.

The golf ball of the invention can be made to conform with the Rules of Golf for competitive play, and may be formed to a diameter of not less than 42.67 mm. It is suitable to set the weight to generally not less than 45.0 g, and preferably not less than 45.2 g, but not more than 45.93 g.

As described above, in this invention, it is possible to substantially reduce the distance traveled by the ball on high HS shots while at the same time holding down as much as possible the decrease in distance traveled on low HS shots. As a result, a superior golf ball for competitors having a low head speed can be obtained.

EXAMPLES

The following Examples and Comparative Examples are provided by way of illustration and not by way of limitation.

Examples 1 to 3, Comparative Examples 1 to 5

The rubber compositions shown in Table 1 were prepared, then molded and vulcanized at 155° C. for 15 minutes to produce solid cores. Numbers in the table indicate parts by weight.

TABLE 1 A B C Polybutadiene rubber 1 100 100 0 Polybutadiene rubber 2 0 0 100 Zinc acrylate 30.0 31.5 28.0 Peroxide 1.2 1.2 1.2 Zinc oxide 4 4 4 Barium sulfate 20.1 21 18.7 Calcium carbonate 0 0 5 Antioxidant 0.1 0.1 0.1 Zinc salt of pentachlorothiophenol 0.6 0.6 0 Specific gravity 1.186 1.194 1.186

Trade names of the materials in the table are as follows.

-   Polybutadiene rubber 1:     -   Available under the trade name “BR 730” from JSR Corporation. -   Polybutadiene rubber 2:     -   Available under the trade name “BR 01” from JSR Corporation. -   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   Peroxide: 1,1-Bis(t-butylperoxy)cyclohexane; available under the     trade name “Perhexa C-40” from NOF Corporation. -   Zinc oxide: Available from Sakai Chemical Industry Co., Ltd. -   Barium sulfate: Available under the trade name “Precipitated Barium     Sulfate 100” from Sakai Chemical Industry Co., Ltd. -   Calcium carbonate:     -   Available under the trade name “Silver W” from Shiraishi Calcium         Kaisha, Ltd. -   Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi     Shinko Chemical Industry Co., Ltd.

Next, the cover material shown in Table 2 below was injection molded over the above core, thereby obtaining a multi-piece solid golf ball in which the core is encased within an inner cover layer and an outer cover layer of given thicknesses.

TABLE 2 Cover material (pbw) D E F Himilan 1605 69 0 0 Dynaron 6100P 31 0 0 Pandex T8290 0 25 0 Pandex T8295 0 75 100 Calcium hydroxide 2.3 0 0 Polytail H 2 0 0 Behenic acid 18 0 0 Polyisocyanate compound 0 9 9 Thermoplastic elastomer 0 15 15 Titanium oxide 0 3.5 3.5 Polyethylene wax 0 1.5 1.5

Trade names of the materials in the table are as follows.

-   Himilan: An ionomeric resin available from DuPont-Mitsui     Polychemicals Co., Ltd. -   Dynaron 6100P: A hydrogenated polymer available from JSR     Corporation. -   Pandex: MDI-PTMG type thermoplastic polyurethanes available from DIC     Bayer Polymer. -   Calcium hydroxide:     -   Available under the trade name “CLS-B” from Shiraishi Calcium         Kaisha, Ltd. -   Polytail H: A low-molecular-weight polyolefin polyol available from     Mitsubishi Chemical Corporation. -   Behenic acid: Available under the trade name “NAA-222S” from NOF     Corporation. -   Polyisocyanate compound:     -   4,4′-Diphenylmethane diisocyanate. -   Thermoplastic elastomer:     -   Available under the trade name “Hytrel 4001” from DuPont-Toray         Co., Ltd. -   Titanium oxide: Available under the trade name “Tipaque R550” from     Ishihara Sangyo Kaisha, Ltd. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd.

Simultaneous with injection molding of the cover, numerous dimples were formed on the surface of the cover, after which the cover was spray-painted. In each example and comparative example, the dimples on the surface of the ball after painting satisfied the parameters shown in Tables 3 to 8 below. In these tables, the dimple types designated as Da refer to dimples having a diameter of 3.7 mm or more, and the dimple types designated as Db refer to dimples having a diameter of less than 3.7 mm.

With regard to the dimple patterns in the tables, the dimple pattern for Examples 1 and 3 is shown in Table 3 (FIG. 3), the pattern for Example 2 is shown in Table 4 (FIG. 4), the pattern for Comparative Examples 1 and 5 is shown in Table 5 (FIG. 5), the pattern for Comparative Example 2 is shown in Table 6 (FIG. 6), the pattern for Comparative Example 3 is shown in Table 7 (FIG. 7), and the pattern for Comparative Example 4 is shown in Table 8 (FIG. 8). These diagrams are all top views of the ball. In the respective examples, the bottom views of the ball have the same pattern as the top views, and are thus omitted.

TABLE 3 Examples 1 Number and 3 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 40 4.1 0.21 1.53 Da-II 184 3.9 0.20 1.31 Db-I 96 3.3 0.16 0.73 Da-III 32 4.1 0.23 1.72 Da-IV 16 3.9 0.22 1.45 Db-II 16 3.2 0.15 0.62 Db-III 8 3.2 0.14 0.49

TABLE 4 Number Example 2 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 24 4.5 0.16 1.34 Da-II 150 4.3 0.16 1.22 Da-III 66 3.7 0.15 0.86 Db-I 18 2.7 0.12 0.36 Db-II 6 2.5 0.12 0.31 Da-IV 48 4.3 0.17 1.30 Da-V 12 3.8 0.16 0.96 Db-III 6 3.4 0.16 0.75 Db-IV 6 3.3 0.15 0.66

TABLE 5 Comparative Number Examples 1 and 5 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 24 4.7 0.15 1.25 Da-II 168 4.5 0.15 1.15 Da-III 48 3.9 0.15 0.85 Db-I 12 2.9 0.15 0.44 Db-II 12 2.6 0.11 0.24 Da-IV 30 4.4 0.16 1.20 Da-V 36 3.9 0.17 0.94 Db-III 8 3.5 0.16 0.70 Db-IV 6 3.4 0.15 0.61

TABLE 6 Comparative Number Example 2 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 12 4.6 0.16 1.28 Da-II 222 4.4 0.16 1.16 Da-III 36 3.8 0.15 0.80 Db-I 12 2.6 0.12 0.58 Da-IV 12 4.4 0.17 0.25 Da-V 24 3.8 0.16 1.25 Db-II 6 3.5 0.16 0.86 Db-III 6 3.4 0.15 0.7

TABLE 7 Comparative Number Example 3 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Da-I 228 4.3 0.17 1.06 Da-II 36 3.7 0.16 0.74 Db-I 12 2.5 0.12 0.23 Db-II 12 3.4 0.17 0.72 Da-III 42 4.3 0.18 1.14 Da-IV 24 3.7 0.17 0.80 Da-V 12 4.3 0.17 1.05 Da-VI 2 3.9 0.16 0.89

TABLE 8 Comparative Number Example 4 of Diameter Depth Volume Dimple type dimples (mm) (mm) (mm³) Db-I 114 3.65 0.196 1.071 Da-I 114 4.0 0.153 1.013 Db-II 60 3.65 0.195 1.071 Db-III 12 2.5 0.167 0.431 Db-II 60 4.0 0.153 1.013

Various properties of the resulting multi-piece solid golf balls were investigated as described below. The results are shown in Table 9.

Deflection of Solid Core and Finished Ball

The solid core and the finished ball were placed on a hard plate, and the deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured.

Cover Hardness (Shore D Hardness)

The cover-forming material was formed under applied pressure to a thickness of about 2 mm and the resulting sheet was held at 23° C. for 2 weeks, following which the Shore D hardness of the sheet was measured in accordance with ASTM D2240.

CL Ratio

The ratio of the coefficient of lift CL of a ball launched using an Ultra Ball Launcher (UBL) at a Reynolds number of 70,000 and a spin rate of 2,000 rpm with respect to the coefficient of lift CL of a ball launched at a Reynolds number of 80,000 and a spin rate of 2,000 rpm was calculated.

Initial Velocity

The initial velocity of the ball was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The ball was held isothermally in a 23±1° C. environment for at least 3 hours, then tested in a chamber at a room temperature of 23±2° C. The ball was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). One dozen balls were each hit four times. The time taken for the ball to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity (m/s) of the ball. This cycle was carried out over a period of about 15 minutes.

Flight Performance

A W#1 was mounted on a swing robot, and the distance traveled by the ball when hit at a head speed (HS) of 54 m/s or 35 m/s was measured. The club used was a TOURSTAGE X-DRIVE 701 (2009 model; loft angle, 9.5°) manufactured by Bridgestone Sports Co., Ltd.

TABLE 9 Example Comparative Example 1 2 3 1 2 3 4 5 Core Formulation A A B C C C C A Diameter (mm) 37.3 37.3 36.5 37.3 37.3 37.3 37.3 37.3 Deflection (mm) 3.9 3.9 3.5 3.9 3.9 3.9 3.9 3.9 Inner Material D D D D D D D D Cover Hardness (Shore D) 56 56 56 56 56 56 56 56 Layer Thickness (mm) 1.7 1.7 2.1 1.7 1.7 1.7 1.7 1.7 Outer Material E E E E E E E E Cover Hardness (Shore D) 55 55 57 55 55 55 55 55 layer Thickness (mm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Cover hardness difference (Shore D) 1 1 −1 1 1 1 1 1 (inner layer − outer layer) Dimples Number of dimple types 7 types 9 types 7 types 9 types 8 types 8 types 5 types 9 types Number of dimples 392 336 392 344 330 368 360 344 SR value (%) 72 76 72 80 78 76 71 80 VR value (%) 1.20 1.06 1.20 0.90 0.88 0.93 0.90 0.90 Average DP (mm) 0.19 0.18 0.19 0.15 0.15 0.16 0.17 0.15 Average DM/DP 19.83 21.95 19.83 27.39 24.77 23.17 20.97 27.39 (Total number of Db)/ 0.44 0.12 0.44 0.12 0.08 0.07 1.07 0.12 (Total number of Da) Volume proportion of 82 96 82 95 95 97 48 95 Da dimples ( % ) Low-velocity CL ratio (%) 85 78 85 80 78 65 75 80 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 45.3 45.3 45.3 Deflection (mm) 3.0 3.0 2.6 3.0 3.0 3.0 3.0 3.0 Initial velocity (m/s) 77.3 77.3 77.2 76.2 76.2 76.2 76.2 77.3 Flight HS Carry (m) 261.2 262.7 262.2 266.0 266.8 264.8 265.2 271.9 54 m/s Total 275.4 276.8 276.9 276.1 276.7 275.9 275.1 282.9 distance (m) HS Carry (m) 163.1 163.5 163.7 158.6 159.0 160.1 159.3 162.9 35 m/s Total 182.5 181.9 182.3 177.3 177.0 178.1 176.8 181.6 distance (m) Difference in carry (m) 98.1 99.2 98.5 107.4 107.8 104.7 105.9 109.0 Difference in total 92.9 94.9 94.6 98.8 99.7 97.8 98.3 101.3 distance (m)

In the above table, Comparative Examples 1 to 4 are prior-art reduced-distance golf balls, and Comparative Example 5 is a prior-art high-rebound golf ball. Here, on comparing the reduced-distance golf balls of Examples 1 to 3 with those of Comparative Examples 1 to 4, it can be seen that the balls in Comparative Examples 1 to 4, owing to their lower rebound (initial velocity) relative to the prior-art high-rebound golf ball of Comparative Example 5, travel substantially reduced distances (both the carry and the total distance) not only at a high head speed but also at a low head speed. By contrast, it was confirmed that the golf balls in Examples 1 to 3 of the invention, by having the same rebound (initial velocity) as the high-rebound golf ball in Comparative Example 5 and by combining therewith dimples which satisfy specific parameters and can thus achieve a relatively low trajectory, are more effective than the balls in Comparative Examples 1 to 4 at suppressing the reduction in distance when hit at a low head speed relative to the substantial reduction in distance when hit at a high head speed. That is, the golf balls of the present invention were confirmed to be golf balls for which the difference in distance when hit at a high head speed versus when hit at a low head speed is small, and which are thus able to achieve a superior distance in the low head speed range while holding down the distance traveled in the high head speed range. 

1. A multi-piece solid golf ball comprising a solid core, an inner cover layer and an outer cover layer, which outer cover layer has a plurality of dimples on a surface thereof, wherein a value obtained by subtracting a material hardness (Shore D) of the outer cover layer from a material hardness (Shore D) of the inner cover layer is greater than −5 and less than +5; the dimples number at least 250 but not more than 500, have a surface coverage (SR) of at least 70% and a volume ratio (VR) of at least 1.0%, are of at least three types of mutually differing dimple diameter (DM) and/or dimple depth (DP), and have an average depth of at least about 0.18 mm and a diameter-to-depth ratio (DM/DP) of not more than about 23; and the ball has a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is maintained at 60% or more of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000.
 2. The multi-piece solid golf ball of claim 1 wherein, letting Da represent a dimple having a diameter of at least 3.7 mm and Db represent a dimple having a diameter of less than 3.7 mm, the ratio (total number of Db)/(total number of Da) is at least about 0.005 but not more than about
 1. 3. The multi-piece solid golf ball of claim 2, wherein dimples Da of at least 3.7 mm account for at least about 75% of the total dimple volume.
 4. The multi-piece solid golf ball of claim 1, wherein the material hardness (Shore D) of the inner cover layer is from 45 to
 65. 5. The multi-piece solid golf ball of claim 1, wherein the material hardness (Shore D) of the outer cover layer is from 45 to
 65. 6. The multi-piece solid golf ball of claim 1, wherein the value obtained by subtracting the material hardness (Shore D) of the outer cover layer from the material hardness (Shore D) of the inner cover layer is within ±4. 